1. Field of the Invention
The present invention relates to a signal waveform shaping technique, and more particularly, it relates to a contour correcting technique for improving sharpness, fineness, and the like of video signals in a color television receiver, video tape recorder, etc.
2. Description of the Prior Art
Japanese Patent Laying Open Gazette No. 63-292776 discloses a waveform shaping apparatus for correcting a contour in which three video signals, a video input signal and two video signals produced by delaying the video input signal in two delay circuits, are utilized so as to improve sharpness, fineness and the like of the video signals. A schematic block diagram thereof is shown in FIG. 64.
An input signal A is delayed by a specified period of time by a delay circuit 1 and by twice as much as the period by delay circuits 1 and 2 to produce signals B and C, respectively. An amplitude difference between the signals A and B is calculated by a subtractor 3 all the time, and further, its absolute value is calculated by an absolute value circuit 5. In other words, signal D output by the absolute value circuit 5 is an absolute value of the difference between the signals A and B. An amplitude difference between the signals B and C is similarly calculated by a subtractor 4 all the time, and its absolute value is further calculated by an absolute value circuit 6. In other words, a signal E output by the absolute value circuit 6 is an absolute value of the difference between the signals B and C.
A subtractor 7 finds an amplitude difference between the signals D and E, that is, signals Fa(=D-E) and Fb(=E-D) are calculated. A comparator 47 compares the signals Fa and Fb in magnitude with predetermined threshold voltage V.sub.TH to produce a switch signal to a switch 48. The switch 48 simultaneously receives the input signal A, the signal B delayed by the specified period, and the signal C delayed by twice as much as the specified period and switches between the signals A, B and C in accordance with the switch signal so as to produce an output signal Na.
FIG. 65 shows the relationship of the signals A, B, C, D, E, Fa, Fb and Na on time-varying basis. There provided is a case where the input signal A exhibits a waveform in which it begins to linearly rise at time t1 and reaches a specified value V.sub.A at time t.sub.3, i.e., a case where a period (t.sub.3 -t.sub.1) is required for rising.
The signal B is obtained by delaying the signal A by a period (t.sub.3 -t.sub.1)/2 in the delay circuit 1. Specifically, the signal B exhibits a waveform in which it begins to linearly rise at time t.sub.2 =(t.sub.3 +t.sub.1)/2 and reaches the specified value V.sub.A at time t.sub.4 =(3t.sub.3 -t.sub.1)/2.
The signal C is obtained by delaying the signal B by a period (t.sub.3 -t.sub.1)/2 in the delay circuit 2. Specifically, the signal C exhibits a waveform in which it begins to linearly rise at time t.sub.3 and reaches the specified value V.sub.A at time t.sub.5 =2t.sub.3 -t.sub.1. Thus, waveforms of the signals D(=ABS(A-B)), E(=ABS(B-C)), Fa(=D-E) and Fb(=D-E) are shaped as shown in FIG. 56, respectively.
The comparator 47 compares the predetermined threshold voltage V.sub.TH with the signals Fa and Fb in magnitude to produce a switch signal which shows a comparison result. A switch 48 selects the signal C for the period when the comparison result satisfies Fa&gt;V.sub.TH, selects the signal A for the period of Fb&gt;V.sub.TH, or selects the signal B in any other conditions. As a result, the waveform shaped output signal Na which requires only a short period of time close to the time t.sub.3 for rising can be obtained.
Since a conventional waveform shaping apparatus is configured as previously mentioned, the output signal Na cannot be obtained when the input signal A which remains stationary for a short period compared with delay times caused by the delay circuits 1 and 2 is to be shaped in waveform, as shown in FIG. 66.
Ideally, it is desirable to obtain the output signal Na which sharply rises close to the time t.sub.3, takes a constant value V.sub.A until the time t.sub.5, and sharply falls close to the time t.sub.5.
However, when a signal (A-B) (shown by broken line in the waveform expressing the signal D) and a signal (B-C) (shown by broken line in the waveform expressing the signal E) are different in polarity from each other, that is, from time t.sub.30 till time t.sub.40, undesirable signal processing is performed, and there arises the problem that the signal Na exhibits a waveform considerably different from that of the input signal A.
Especially, as shown in FIG. 67, this problem becomes more serious as a sensitivity of the comparator 47 is enhanced to make the output signal Na sharply rise close to the time t.sub.3 and sharply fall close to the time t.sub.5. This means a restriction is imposed on the delay times of the delay circuits 1 and 2 and the sensitivity of the comparator 47 in the conventional waveform shaping apparatus.
Moreover, the switch 48 switches between the signals A, B and C to produce an output signal in the conventional waveform shaping apparatus, and hence, there also arises a problem that an amount of waveform shaping cannot be freely determined.